1. Field of the Invention
The present invention relates to a color electrophotographic image forming apparatus adopting a rotary developing process.
2. Description of the Related Art
Full-color image forming apparatuses each including a single image bearing member and a rotatable support integrally supporting multiple developing units are heretofore known. The full-color image forming apparatuses adopt a developing process in which the developing units are sequentially switched at certain timing to develop an electrostatic latent image formed on the surface of the image bearing member.
Such an image forming apparatus that uses a rotatable developing unit support (rotary) integrally supporting multiple developing units and that sequentially switches the developing units to develop an electrostatic latent image on the surface of a single image bearing member is referred to as a rotary image forming apparatus.
Japanese Patent Laid-Open No. 2005-148319 discloses the structure of a rotary image forming apparatus in related art. In general, in a developing process using a rotary (hereinafter referred to as a rotary developing process), it is necessary to perform an operation in which developer bearing members in the developer units corresponding to developer of the respective colors are sequentially in contact with and isolated from the surface of the image bearing member at a developing position. The switching of the developing units is performed by the developing unit support that rotates while the developing units are isolated from the surface of the image bearing member.
The image forming apparatus in the related art adopts a structure in which the developing unit support is moved in the radial direction (the direction of the center of the rotation) of the image bearing member by using, for example, a cam having a driving force to bring each developer bearing member in contact with the surface of the image bearing member or isolate each developer bearing member from the surface of the image bearing member.
However, the rotary image forming apparatus in the related art has the following problem.
Since the image forming apparatus in the related art adopts the structure in which the developing unit support is moved in the radial direction of the image bearing member to bring each developer bearing member in contact with the surface of the image bearing member or isolate each developer bearing member from the surface of the image bearing member, it is necessary to prepare a space in which the developing unit support is moved.
In addition, it is necessary to provide, for example, a cam serving as a driving unit that moves the developing unit support to the radial direction of the image bearing member in conjunction with the contact and isolation operation.
In other words, it is necessary to provide the space and the driving unit for moving the entire developing unit support to the radial direction of the image bearing member to bring each developer bearing member in contact with or isolate each developer bearing member from the surface of the image bearing member in the rotary image forming apparatus in the related art. Accordingly, it is difficult to achieve a reduction in size and cost of the main body of the apparatus.
In order to resolve the above problem, the rotation of the developing unit support may be directly used to, for example, cause each developer bearing member to be in contact with the surface of the image bearing member or to be isolated from the surface of the image bearing member without using the driving unit, such as the cam.
However, in the image forming apparatus having such a structure, the developer on the developer bearing member can be unnecessarily transferred to the image bearing member when each developer bearing member passes along the image bearing member due to the rotation of the developing unit support to cause an image defect, such as a streaked image or a spot on the rear face of a sheet of paper.
In order to prevent such an image defect, it is necessary to produce an electric field for preventing the developer from being transferred to the image bearing member between the surface of the image bearing member and the surface of each developer bearing member when the developer bearing member passes along the image bearing member.
In the general rotary image forming apparatus, the contact between an electrical contact of the main body of the image forming apparatus and an electrical contact of each developing unit, at which a developing bias is applied, and the isolation of the main body of the image forming apparatus from the electric contact are repeated in conjunction with the rotation of the developing unit support.
The present invention provides an image forming apparatus, such as a rotary image forming apparatus, capable of generating an excellent image without causing the malfunction of the image forming apparatus and the damage of the electrical contacts.